Anodising treatment for aluminium



United States Patent 3,468,772 AN ODISING TREATMENT FOR ALUMINIUM PeterGeoffrey Sheasby, Banbury, Edward Percival Short, North Aston, and RomanDominik Guminski, Banbury, England, assiguors to Aluminium LaboratoriesLimited, Montreal, Quebec, Canada No Drawing. Filed July 8, 1966, Ser.No. 563,691 Claims priority, application Great Britain, July 14, 1965,29,940/ 65 Int. Cl. C23b 9/02 U.S. Cl. 204-58 6 Claims ABSTRACT OF THEDISCLOSURE In procedure for producing coloured coatings on aluminium byanodising the aluminium in an oxalic acid solution, significantly deepercolours are obtained while providing an efiective anodic oxide coating,by performing the anodic treatment with a pulsed direct currentadvantageously having stated characteristics, including defined valuesof the relative peak and means current density and of the pulsefrequency, the pulses preferably being of approximately square waveform.

The present invention relates to the production of anodic oxide films onaluminium, including aluminium alloys and in particular it relates to amethod of producing coloured anodised films.

It is well known that for external uses, such as window frames andcurtain walling panels for buildings, aluminium alloys are oftenprotected by a hard, low porosity, anodic oxide film to preventcorrosion.

It is also well known that colours can be developed in such films duringthe anodising process when certain acids are used in the anodising bath,the intensity and shade of the colour being dependent upon a number offactors, including the actual constituents of the aluminium alloy andthe composition of the anodising bath. Thus it is known that very darkfilms are obtained with aluminium alloys having a high silicon content,and that bronze shades are obtained with many other aluminium alloyswhen the anodising is performed in a bath containing certain sulphonatedorganic acids, such as sulphophthalic acid and sulphosalicylic acid.Although the processes using these acids produce strongly colouredanodic oxide films, nevertheless they are expensive to operate becauseof the high cost of the electrolyte and because certain expensiveauxiliary equipment, such as ion-exchanger columns, are found to benecessary to avoid high electrolyte loss.

The use of oxalic acid as the electrolyte in an anodising process hasalready been described. With oxalic acid anodising, using directcurrent, a range of attractive colours can be developed in the anodicoxide film on certain aluminium alloys, but these are not of the sameorder of intensity (except on certain silicon-containing alloys) as isobtained with the sulphonated organic acids referred to above, operatingunder the same conditions of current density on the same alloys.

It is well known that the hardness of anodic oxide films depends to alarge degree on the temperature at which the anodising process isperformed. Aluminium, bearing an anodic oxide film which has becomecoloured during the anodising process, is mainly used for architecturalpurposes, such as window frames and shop front fittings produced fromextruded aluminium sections. For these purposes it is important that theanodic oxide film should be hard and have low porosity, so that it mayprovide prolonged protection for the metal.

To ensure the production of anodic oxide films of this character it isfound necessary to control the temperature ICC of the acid anodisingbath to a temperature of 30 C. at most; 20 C. being the temperaturecommonly used.

High current densities are employed in anodising baths and thus muchheat is developed in the electrolyte. The temperature, to which theelectrolyte can be held, depends upon the associated equipment forcooling the electrolyte and in practice, the cooling capacity of suchequipment controls the total current capacity of an anodising bath of agiven size. Beyond a certain size the capital cost of the associatedequipment for cooling the electrolyte becomes prohibitive and thus theoutput of an anodising bath in terms of area of anodised aluminium is afunction of the current density. Furthermore, the current density isalso limited by the fact that under D.C. anodising conditions the heatproduced in the pores must be dissipated to avoid burning of the film.

According to the present invention, as compared with normal directcurrent anodising, significant darkening of the colours developed inanodic oxide films may be achieved if the direct current applied to ananodising bath containing oxalic acid is pulsed without change of theaverage current density, so that during each pulse the maximum currentdensity is appreciably increased as compared with operation under steadydirect current conditions. The heat developed in the anodising bath isnot much greater than if a steady direct current were passed, but thedepth of the colour developed is increased on thealuminium-magnesium-silicon alloys conventionally employed in theproduction of extrusions for external use.

Although the anodising bath may contain mineral acid, salts of mineralacids or monocarboxylic acids to improve conductivity, oxalic acid mustform the major proportion of the solute. One of the principal advantagesof this method is that it permits a whole range of colours to bedeveloped in an anodic oxide film of a given thickness by variation ofthe mark/ space ratio of the pulses i.e. the ratio of the period duringwhich the current flows to that Where it is zero or substantially zero,without changing the average current density. The thickness of theanodic oxide film is a function of the product of the average currentdensity and the treatment time.

It is found very desirable to avoid any negative component in the pulsedcurrent, because this leads to soft anodic films.

From studies which we have carried out, we have concluded that thecolours developed in anodic oxide films formed in organic electrolytesare mainly due to optical interference effects arising from the,presence of fine particles dispersed in the anodic aluminium oxide. Wehave postulated that the colours developed in anodic films produced inbaths containing oxalic acid are due, at least in part, to carbonparticles produced by decomposition of the oxalic acid component of theanodising bath. We have further postulated that such decomposition isdue to the development of localised high temperatures, resulting fromhigh resistances in the oxide film at the bottom of the individualpores.

On the basis of these hypotheses it seems probable that deepening of thecolour developed in the anodic oxide film is produced if such localisedhigh temperatures are momentarily increased by pulsing the current,because during the pulses the current density at the surface of thealuminium is increased. Although it might be thought that the sameresult could be obtained by increasing the current density in a standardconstant D.C. anodising process the greater amount of heat generated atthe surface of the metal results in the metal itself becoming overheatedand the occurrence of local burning of the anodic aluminium oxide film.

It is however found that the darkening of the colour developed in theanodic oxide film is not particularly significant unless the pulsefrequency is less than a maximum value and the mark/ space ratio of thepulses is below a maximum value. Thus it is believed that theseconditions are required so that the increased current density at themetal surface is maintained during each pulse for a sufficient timeinterval to enable the local lighter as the peak current density wasdecreased. The time required to produce an anodic film of desiredthickness varied a little from alloy to alloy, but the above time figureis typical.

Alloys HE9 and HEZO are aluminium alloys conventemperature at the bottomof the pores to increase sig- 5 tionally used for production ofextrusions for architecturntificgntly abotvleD Ehe tergperature 1n th1sregion during ?lfi1sage and have a nominal composition respectively as sea y curren ano ismg. o ows:

It is found that the maximum frequency of the p ls HE9Si, 0.6%; Mg.0.45,- balance Al and impurities. is of the order of 50 pulses persecond and preferably 10 HE204i, 05%; 10%; 25%; Cr, 25%; the pulsefrequency is much lower than this, very satisance A1 and impurities t ey results being obtained when the Pulse frequency Alloy HEZO cannot beextruded at the same high rate l ri iirliii tii iiii Zii$$$$ as to haveas if E iiiwere i uce ex ruszons in is a oy. square a Wave form aspossible and the maximum value The peak current density employed can beas high as at lihemarl/slpace rgtrofills about 50% g pft l l ge t 500amps/sq. foot without burning the anodic oxide sta ens; at; that:antenna, :55: as; We If a smoo current were app ie t e y h expressedalternatively y seylhg that the No difiiculties were experienced keepingthe electrolyte mlnllhllm Yatlo Of the P e l h dehslty il a l? at thedesired operating temperature, using refrigeration t0 the e h e dehslty1S Preferably t e that and temperature controls normally available in aDC. P'i t In e range l 1 3;? lhet t g oxal1c ac1d anodising tank for theproduction of hard, z el g l t a r 13 iieo f g yco ege i e ive l;corrosion resistant anodic oxide films for external use. 1e e e m SPaeeT 1 0 0 h e No abnormal signs of electrolyte deterioration were dehettelresults, the cost of the heeessery eqhlpmeht to P tected and theelectrolyte has substantially the same service 2 g y gg gz g gg s g 1SProbably so great life as the oxalic acid electrolyte in the DC. oxalicacid I r 1 u e anodisin rocess.

The minimum mean anode current density is about 10 physicga? examinationof the anodic oxide films amps/ square foot, with a maximum mean currentdensity duced by the use of Pulsed DO curmm with an oxalic of about amPS/Square foot- Above that value the acid anodising bath, operated underthe conditions set t t g g fi t g fi e h gg i forth above, shows nosubstantial difference, except colon r, $2 1; :ich ie v d Altl' rorf lit iie u e (if hi hei' current be-tveen E filn-ls and thosehpgxglced by zg e e aci ano 1s1n usm a smoot current. cce rate density leadsdevelopment of darker Polours corrosion test s hav e shown no differencein corfosion z gz q f g gi 22 352212 21 ggfigig range 15 resistancebetween anodic oxide films obtained using In one series of tests the DC.current-pulses were $15: Dc and Smooth other vanable bemg the generatedby using thyristors in a three-phase half-wave rectifier system, thefiring of the thyristors being coni f 6 2 5 i by R is trolled by meansof a variable mark/space ratio pulser, 40 i ar m mate at t e co ours aveSans which controls the time interval during which the thyris toryhgasmess' tor remains conductive, the pulser also having a variableAccordmg to a further dftvelopment of the mlfentlon repetition period.it has been found possible in some cases to obta n very It was foundthat a considerable rang: of colours dark colours in anodic filmsproduced in an oxalic acid could be obtained on aluminium and aluminiumalloys, t 15mg 3 relatlvely small Peak current/mean current whenanodised in oxalic acid solution, by variation of the l'atlo of Order P611 y treatment the metal t mark/space ratio and repetition period(pulse fredevelop the formation of fine particles which survive 1nquency). In addition to those factors, the colour obtained the anodicxide film. with an anodic film of a given thickness, depends on the Inthe case of the alloy HE9, referred to above, good alloy selected fortreatment, the mean current density results can be obtained byoverageing, that is to say by applied and the temperature of theelectrolyte carrying out the normal artificial ageing heat treat- It ispreferred that the temperature of the oxalic acid ment f a longer i d/at a hi h temperature electrolyte should be in the range of l5-3() C.and that tha n 1s required for the development of the optimum theaqueous electrolyte should contaln at least 8%doxa1l1c physicalpropel.tigcs The Overageing heat treatment is f and Shou d prefefab yCommute? e f lieved to increase the development of particles of thetion. In accordance with known pract1ce, it is desirable Mgzsitype toapply a reduced voltage at the beginning of the anodisa- 1, tion processto avoid uneven colour distribution; applicaa i i z; for i 2 i heat;tion of full process voltage at the beginning of the treatmg at fi yslgfn f H ar F i o ment leads to greater intensity of colour around theedges 60 the rfsultant ozude 1S obtanted If t e a CY 15 0V6!" of thepiece than in the middle of the Surface aged for a sultable tlme at ahigher temperature, when In a series of tests carried out using asaturated oxalic helhg Pulsed euflzehtsatlstaetofy results are acidanodising bath at 20 C. and a mean current density tamed 1f g g 15carried ut at 220i5 C. for 10 of 24 amps/sq. ft., the following processconditions hours. Although this somewhat decreases the physical wereused: properties of alloy extrusions, it is not to such extent Mark/Peak Peak Voltage space current Pulse frequency ratio, density AlloyAlloy Time lfilm (cycles/second) percent (a./sq.ft.) HE9 H1320 (ruins)(microns) The first conditions given produced the darkest colour thatthe extrusions become unacceptable on strength on all alloys tested. Thecolours became progressively 75 grounds. These treatment conditions havebeen selected .5 since they are foundto produce reproducible colours, inspite of minor operational changes of the treatment conditions.

The use of overaged" material allows the process to be operated with aless expensive source of pulsed D.C.

current, capable of a maximum peak'current/rnean cur- 5 rent ratio of 6:1.

Using such apparatus and anodising to a film thickness of 25-35 microns,generally considered necessary for external application of aluminium inthe English climate we have produced the following range of colours onHE9 10 alloy overaged at 220 C. for 10 hours.

Mean Approxi- Peak to current Anodicfilm Anodising Electrolyte at Samplemean curdensityin thickness time in temperature Peak mean ak No.rentratio amps/sq. it. inmicrons min. 111C. voltage vo tage Colour 6:124 35 60 20 115-160 137 Black. 6:1 25 55 105-145 125 Brown-bronze. 3:115 55 20 90-130 110 Bronze. 6:1 24 25 105-155 130 Brown-bronze. 6:1 1525 25 100-135 117 Bronze. 3:1 15 25 55 25 -120 Light bronze. 6:1 24 3550 30 100-125 112 Brown. 6:1 15 25 55 30 90-120 Gold-brown. 3:1 15 25 5530 Gold-bronze.

The following table shows the mechanical properties of HE9 alloyaveraged under varying conditions.

We have so far exemplified the process with reference to anodising bathscontaining oxalic acid alone.

Oxalic acid may have employed in conjunction with it a small proportionof another acid, either an inorganic acid, such as sulphuric acid, or anorganic acid, such as formic acid, to improve the conductivity of theelectrolyte. Alternatively a metal salt, such as ferrous sulphate, maybe used for the same purpose. Such addition in the case of sulphuricacid or ferrous sulphate is not in excess of 1% and preferably much lessso as not to interfere with the type of film formed by anodising inoxalic acid.

The table below summarises the results Obtained using the pulse currentanodising technique in various mixed electrolytes containing oxalicacid.

The same HE9 alloy was used for all the experiments and the anodisingconditions used were kept constant at a mean current density of 24a./sq. ft. and a time of 40 55 mins. A peak/mean current density ratioof 6:1 was used for the pulsed conditions.

All percentages for the electrolyte components are on a Weight to volumebasis.

in an aqueous electrolyte bath containing a solute of which a majorproportion is oxalic, acid and passing through said bath a pulsed directcurrent, in which the wave form of each current pulse is approximatelysquare and the ratio of anode peak current density/anode mean currentdensity is 36:1.

2. A process for producing a coloured anodic oxide coating on aluminium(including aluminium alloys) comprising suspending the material to becoated as an anode in an aqueous electrolyte bath containing a solute ofwhich a major proportion is oxalic acid and passing through said bath apulsed direct current, in which the pulse frequency is 1 /z-20 pulsesper second and the ratio of anode peak current density to anode meancurrent density is at least 3 1.

3. A process according to claim 2, in which the wave form of eachcurrent pulse is approximately square.

4. A process according to claim 3, in which the mean anode currentdensity is 10-36 .amperes per square foot.

'5. A process according to claim 3, in which the bath is a substantiallysaturated aqueous solution of oxalic acid.

6. A process according to claim 3, in which the material to be anodisedis an aluminium alloy which consists essentially of about 0.6% Si" andabout 0.45% Mg, balance aluminium and impurities, and for which a normalageing treatment is heating at C. for 8 hours, said process furthercomprising, as a step preceding the aforesaid anodic treatment,subjecting the aforesaid alloy to a heat ageing treatment which in atleast one, of the respects very patchy film.

It will be seen from these results that a darkening of the film isobtained by the use of the pulsed current technique only in those caseswhere the character of the an- 75 of temperature and time exceeds theaforesaid normal ageing treatment.

(References on following page) 7 8 References Cited FOREIGN PATENTSUNITED STATES PATENTS 716,554 10/1954 Great Britain.

1,735,286 11/1929 Kujirai et a1 20458 JOHN MACK, Primary Examiner2,920,018 1/ 1960 Miller 204-56 5 R. L. ANDREWS, Assistant ExaminerUNITED STATES PATENT OFFICE CILRTLFICATE OF CORRECTIUN Patent 3,468, 772Dated September 23 1969 PETER GEOFFREY SlIEASBYf EDWARD PERCIVAL SHORTand Inventor(s) DQMIAII'K GHMINSKI it is certified that error appears inthe above-identified PGLGUC and that said Letters Patent are herebycorrected as shown below:

Column 4, line 10, "Cu. 25%; Cr, 25%" should read --Cu 0.25%; Cr 0 2 S Lslum AND SEALED -Em a orr mmmx.-. m. L, 1m Oomissiom of Patents-

